Project Summary/Abstract Chronic liver disease (CLD) and cirrhosis is the 12th leading cause of death in the United States. Currently, the only treatment for patients with end-stage liver disease is a liver transplant. However, the scarcity of donor organs makes this option untenable for many patients. Therefore there is a great need to develop new therapies to prevent or reverse patient progression to end-stage liver disease. Progression is thought to be due in part to failed hepatic regeneration. Under most circumstances, after liver injury hepatic repair is mediated by proliferation of hepatocytes. However, when hepatocyte proliferation is impaired, liver progenitor cells (LPCs) arise from the biliary epithelial cell (BEC) compartment, expand, and differentiate into hepatocytes. LPC expansion is observed in human CLD patients, and thus LPCs represent a promising target for therapies aimed at promoting liver regeneration (LR) in patients. However the role of LPCs in LR remains controversial. A common model to study LPCs in rodents is the choline deficient, ethionine-supplemented (CDE) diet, which induces liver injury and promotes expansion of LPCs. However, the CDE diet does not block hepatocyte proliferation, so LR is hepatocyte-driven in the CDE diet model. The ?-catenin signaling pathway plays an important role in LR by promoting hepatocyte proliferation. Thus we hypothesize lack of ?-catenin in hepatocytes would impair hepatocyte proliferation after CDE diet-induced liver injury and promote LPC- mediated LR. In Aim 1, we will test this hypothesis using two models of genetic fate tracing in mice. In the first model, we will perform negative lineage tracing by injecting mice with adeno-associated virus serotype 8 (AAV8) carrying Cre recombinase to simultaneously delete ?-catenin in hepatocytes and label hepatocytes with EYFP. In the second method we will perform positive lineage tracing, utilizing Foxl1-Cre mice to label LPCs with EYFP and injecting these mice with ?-catenin small interfering RNA conjugated to a hepatocyte- targeting ligand to knockdown ?-catenin expression specifically in hepatocytes. We will place both mouse models on the CDE diet to determine if LPCs give rise to hepatocytes to mediate LR. The mechanisms of LPC differentiation to hepatocytes are also not understood. Due to the important role of ?- catenin in hepatocyte maturation in development, we hypothesize ?-catenin is important for LPC-to-hepatocyte differentiation. In Aim 2, we will test this hypothesis in vivo through placing mice with lack of ?-catenin in both hepatocytes and BECs (the origin of LPCs) on the CDE diet, where we would expect to observe a defect in LR. We will test this hypothesis in vitro utilizing the small cholangiocyte cell line (SMCC), an immortalized BEC line which expresses LPC-marker Foxl1. We will treat SMCCs with HGF, EGF, and other factors to induce differentiation to hepatocyte-like cells, and we predict ?-catenin-inhibited SMCCs will fail to differentiate. In summary, our work will thoroughly describe the role of ?-catenin in LPC-mediated LR, potentially identifying a new mechanism which could be targeted to promote LR in human CLD patients.